Fuel cell systems are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy into electrical energy. Proton exchange membrane fuel cells comprise an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane and are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product.
Proton exchange membranes require a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive. It has been suggested that each proton that moves through the membrane drags at least two or three water molecules with it (U.S. Pat. No. 5,996,976). U.S. Pat. No. 5,786,104 describes in qualitative terms a mechanism termed “water pumping”, involving the transport of cations (protons) with water molecules through the membrane. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, at least on the anode side, and its internal resistance increases. It will be appreciated that this mechanism drives water to the cathode side, and additionally the water created by reaction is formed at the cathode side. Nonetheless, it is possible for the flow of gas across the cathode side to be sufficient to remove this water, resulting in drying out on the cathode side as well. To maintain membrane conductivity, the surface of the membrane must remain moist at all times. Therefore, to ensure adequate efficiency, the process gases must be, on entering the fuel cell, at an appropriate humidity and at a suitable temperature for keeping the membrane moist. The range for suitable humidities and temperatures will depend on system requirements.
A further consideration is that there is an increasing interest in using fuel cells in transport and like applications, e.g. as the basic power source for cars, buses and even larger vehicles. Automotive applications are quite different from many stationary applications. For example in stationary applications, fuel cell stacks are commonly used as an electrical power source and are simply expected to run at a relatively constant power level for an extended period of time. In contrast, in an automotive environment, the actual power required from the fuel cell stack can vary widely. Additionally, the fuel cell stack supply unit is expected to respond rapidly to changes in power demand, whether these be demands for increased or reduced power, while maintaining high efficiencies. Further, for automotive applications, a fuel cell power unit is expected to operate under an extreme range of ambient temperature and humidity conditions.
All of these requirement are exceedingly demanding and make it difficult to ensure a fuel cell stack will operate efficiently under all the possible range of operating conditions. While the key issues are ensuring that a fuel cell power unit can always supply a high power level and at a high efficiency and simultaneously ensuring that it has a long life, accurately controlling humidity levels within the fuel cell power unit is necessary to meet these requirements. More particularly, it is necessary to control humidity levels in both the oxidant and fuel gas streams. Most known techniques of humidification are ill designed to respond to rapidly changing conditions, temperatures and the like. Many known systems can provide inadequate humidification levels, and may have high thermal inertia and/or large dead volumes, so as to render them incapable of rapid response to changing conditions.
An apparatus and method of controlling temperature and humidity in fuel cell systems is disclosed in the applicant's co-pending U.S. patent application Ser. No. 09/801,916. The method comprises: humidifying a fuel cell process gas stream at a first temperature so as to provide the process gas stream with excess humidity, cooling the process gas stream at a second temperature, lower than the first temperature, to cause condensation of excess moisture, removing excess condensed moisture from the process gas stream and delivering the process gas stream at a known temperature, whereby the relative humidity level in the process gas stream is determined from the ratio of the saturation pressures of the second and the said known temperatures. Particularly, the method includes recovering humidity from the exhausted process gas generated by the fuel cell and using the recovered moisture to humidify at least one of the incoming fuel and oxidant streams. However, this method requires a large number of components and hence reduces the overall efficiency of the fuel cell system.
Another method is disclosed in U.S. Pat. No. 6,013,385. In this patent, a fuel cell gas management system is disclosed. The system comprises: a first reactant humidification subsystem for supplying a first reactant inlet stream to the first reactant inlet of the fuel cell and receiving a first reactant exhaust stream from the first reactant outlet of the fuel cell, said first reactant humidification subsystem comprising an enthalpy wheel for collecting moisture from the first reactant (oxidant) exhaust stream and transferring a portion of the collected moisture to the first reactant inlet stream; a second reactant (fuel) humidity retention subsystem comprising a recirculation loop for collecting excess second reactant from the second reactant outlet of the fuel cell, a source of second reactant mixing means for mixing second reactant from a reactant source with second reactant collected from the second reactant outlet of the fuel cell and motive means for circulating second reactant in said recirculation loop and for introducing second reactant into the second reactant inlet of the fuel cell. However, this patent still fails to fully utilize the waste heat and humidity from fuel cell exhaust. There remains a need for a fuel cell gas management system that can offer rapid dynamic control of temperatures and relative humidities for incoming fuel cell process gases.
Although humidification of reactant streams is a major concern of fuel cell system designers, flooding is also a problem that frequently occurs in fuel cells since reactant streams are usually humidified and water is a product of fuel cell reaction. When one or more cell is flooded in a fuel cell stack, cell voltage of the flooded cell drops and hence the power output of the fuel cell stack decreases. Flooding significantly affects the performance of a fuel cell power system and may lead to permanent damage to the fuel cell stack. Therefore, it is desirable to provide a method of alleviating the flooding problem of fuel cell systems.